The present invention relates to a highly purified metal suitable as a target material for the formation of electrode, contact part and barrier layer of a semi-conductor device, a method for the manufacture of the same and a sputtering target using the same.
As the material to form wiring layer and electrode of a semi-conductor device like LSI, a silicide of a metal having a high melting point such as Mo, W, Ta, Ti, Zr and Hf have been used. Further, because there is a tendency of furthering high integration of semi-conductor devices and calling for more minute wiring structures, the possibility of various problems has been watched with misgivings.
In the case of, for example an Al wiring, minute wiring and higher current density required therefor give rise to electromigration that Al atoms are carried away in the direction of the electron movement or an increase of working heat. These lead to a problem that Al wirings are susceptible to burn-out. Further, an increase in wiring resistance due to minute wiring materials causes delay of signals. Thus, particularly Ti silicides have attracted attention as the wiring and electrode material because they not only have a high melting point but also a low resistance.
For example, when a Ti silicide is intended for use as the material for electrode, a Ti film is formed over a polysilicon film by the sputtering process. Then the Ti film is heat-treated to make a slicide out of Ti. What is called a polycide structure prepared in this way has been put to use. On the other hand, an attempt has been made to make contact parts of a Ti silicide as the built-in protection in order to reduce contact resistance. Further, a contact part is inlaid with, for example a Ti film serving as a diffusion barrier layer in order to prevent Si from precipitating into Al wirings. In this sense, a laminated structure of Al/TiN/TiSi.sub.2 has been used. A TiN film is formed of a reactive sputtering material and the like.
As stated above, a TiN film or a TiSi.sub.2 film is made by the sputtering process. Therefore, the sputtering target required for the process must be made of a Ti material. It is important that the Ti target for that purpose has a high purity. When a Ti target contains an impurity of oxygen, the film formed therefrom has high electric resistance, causing a trouble of delay of signals or burn-out of wirings. Further, a heavy metal such as Fe, Ni and Cr collects on the interface of a laminated film and form a deep level, causing leakage at the joint thereof. An alkali metal such as Na and K readily sets Si free, deteriorating the properties of the device.
Meanwhile, a Ti material for the above mentioned Ti target generally is manufactured according to the so-called Kroll method or Hunter method to thermally reduce a Ti compound such as TiCl.sub.4 with an active metal such as Na and Mg or the fused salt electrolysis process using a fused salt consisting of alkaline metal compound. In recent years when metal purification technology has advanced and manufacturing processes have been well managed, incorporation of impurities such as heavy metal has been brought down to a very low level.
However, even in the case where a TiSi.sub.2 film for use in contact parts is formed of a Ti target having a very low content of such impurities as heavy metal, there is a problem that leakage at the junction cannot be sufficiently prevented because of very minute wiring. This makes an important cause of the loss of fidelity of semi-conductor devices. Further, in a barrier layer, an increase of current density which is caused by reduction of wiring width, leading to malfunction of the layer and leakage at the junction. It is expected that these problems will be exacerbated in the future as integration of semi-conductor devices advances. These problems occur not only when electrodes and barrier layers are made of Ti compounds but also when they are made of Zr and Hf compounds.